This invention deals with a particular kind of dispersed system (or colloid) that can be described as a collection of small particles immersed in a liquid. These particles can be either solid (dispersions), or liquid (emulsions). Such dispersed systems play an important role in the formulation of a variety of materials such as ceramics, paints, lattices, food products, paper coatings, polymer solutions, etc.
These systems all share a common feature. Because of the small particle size, the total surface area of the particles is large relative to their total volume. Therefore surface related phenomena determine their behavior in various processes. This invention applies to dispersed systems where these surface effects are dominant, corresponding to a range of particle size up to about 10 microns. The importance of these surface effects disappears for larger particles.
One of the most commonly used characteristics of these surface effects is the so-called ζ-potential. It is important because of the relationship between this parameter and the aggregation stability of many dispersions and emulsions. There are several methods with which to characterize this parameter. In dilute systems, microelectrophoresis is the most common tool. In the case of flat surfaces and very large particles, a streaming current method is useful.
This patent deals with concentrated dispersions and emulsions. The only available method with which to characterize ζ-potential in these systems, without dilution, is based on Electroacoustic phenomena. A historical overview of this effect is presented in the recently published book by Dukhin, A. S. and Goetz, P. J. “Ultrasound for characterizing colloids. Particle sizing, Zeta Potential, Rheology”, Elsevier, 2002.
There are several patents that introduce methods and devices based on Electroacoustics. All of them present Electroacoustics as an independent technique that allows us to characterize ζ-potential without additional measurements, and in particular without conductivity measurement.
For instance, in U.S. Pat. No. 5,059,909 R. W. O'Brien commented on (page 10, line 60) the “Determination of particle size and electric charge” and claimed that “ . . . Thus by measuring the pressure difference and short-circuit current it is possible to determine (zeta potential) without the need for conductivity measurement.”
In the two other US patents by R. W. O'Brien, U.S. Pat. No. 5,616,872 “Particle size and charge measurement in multi-component colloids” and D. W. Cannon, R. W. O'Brien, U.S. Pat. No. 5,245,290 “Device for determining the size and charge of colloidal particles by measuring electroacoustic effect” there is no discussion whatsoever about the need for conductivity data.
The two earlier US patents by T. Oja, G. L. Peterson and D. W. Cannon U.S. Pat. No. 4,497,208 “Measurement of electro-kinetic properties of a solution” and by A. J. Babchin et al U.S. Pat. No. 5,293,773, “Method for determining the wetting presence of particulate solids in a multiphase liquid system”, also ignore conductivity measurements.
There are claims in several publications that the absence of any need for conductivity data in extracting the ζ-potential from the measured Electroacoustic signal is an important advantage of a particular Electroacoustic method. For example, R. W. O'Brien, B. R. Midmore, A. Lamb and R. J. Hunter wrote in “Electroacoustic studies of moderately concentrated colloidal suspensions”, Faraday Discuss.Chem. Soc., 90, 1-11 (1990): “ . . . From Eq.5 (ESA∝CVP*K*) it can be seen that the ESA is independent of the complex conductivity of the suspension, so the ESA is more convenient than CVP for determining the mobility. The main motivation for determining the mobility is, of course, to obtain information about the particles . . . ”. Another example is review by R. J Hunter “Recent developments in the electroacoustic characterization of colloidal suspensions and emulsions”, Colloids and Surfaces, A, 141, p.37-65, (1998), which make statement on the page 40 that “ . . . measurement of ESA effect gives μd immediately, whereas one needs not only CVP, but also the complex conductivity to obtain μd by the alternate route . . . ”.
There is a reference to the earlier work by R. W. O'Brien given by D. W. Cannon, who contributed to the “New developments in electroacoustic methods and instrumentation” NIST, 1993: “ . . . O'Brien {J. Fluid Mech. 1988} has pointed out that the electrical response at the electrodes can also be measured as the short circuit current which has the advantage that the current is directly proportional to the charge of the particles while the CVP is proportional to the charge divided by the electric conductivity of the suspension”.
A few years earlier the same statement had been made by E. E. Isaacs, H. Huang, A. J. Babchin and R. S. Chow, in “Electroacoustic method for monitoring the coalescence of water-in-oil emulsions”, Colloids and Surfaces, 46, (1990), p. 181:” As shown by O'Brien, the measured pressure will not depend on the complex conductivity of the colloidal system, and therefore this mode of measurement can be recommended when working with colloids where the water phase is continuous. When working with oil continuous colloids, however, CVP mode of operation is preferred. The low value of the complex conductivity will provide for a significant ΔΨ even for small values of dynamic mobility. We may say that the small conductivity of the non-polar media acts as a natural amplifier in electrokinetic measurements by ultrasound vibration potential.                     ΔΨ        =                                            φΔρ              ⁢                                                           ⁢                              cV                0                                                    K              *                                ⁢                                    μ              d                        ⁡                          (              ω              )                                                          (        1        )            
where ΔΨ is potential difference between electrodes, φ is volume fraction, Δρ is density contrast between particle and media, K* is complex conductivity, μd is dynamic electrophoretic mobility.”
We should make note that in our previous US patent that covers the electroacoustic method employed in the Dispersion Technology instruments (U.S. Pat. No. 6,109,098 by Dukhin, A. S. and Goetz, P. J. “Method and device for characterizing particle size distribution and zeta potential in concentrated system by means of Acoustic and Electroacoustic Spectroscopy”), we also ignored the need for conductivity measurement. Even more, we considered the Colloid Vibration Current (CVI) mode superior to the Colloid Vibration Potential (CVP) mode specifically because we then believed that the CVI mode would not require conductivity data for calculating ζ-potential.
However, several recent theoretical and experimental developments have forced us to reconsider the relationship between Electroacoustic and Conductivity measurements with regard to ζ-potential characterization. We now realize that there are several situations where the calculation of ζ-potential, from any mode of Electroacoustic signal, is impossible without conductivity data.
There are three modes of Electroacoustics, depending on the driving force: Colloid Vibration Potential (CVP), Colloid Vibration Current (CVI), and ElectroSonic Amplitude (ESA). The first two use ultrasound as a driving force, whereas the third applies an electric field as the driving force.
It has been generally accepted that one could switch from one mode to another in order to avoid the need for a conductivity measurement. For instance, O'Brien suggested switching from CVP to CVI to achieve this purpose.
We have discovered recently that such “mode switching” is sometimes not productive. There are real systems where the Electroacoustic and Conductivity effects are inexorably intertwined such that both must be measured in order to calculate ζ-potential, no matter which mode of Electroacoustic measurement is employed. We will demonstrate this fact for two important categories of dispersions and emulsions:                Low conducting systems with a thick Double Layer        Dispersions containing particles with high dielectric permittivity        
Unraveling this complex relationship between the electroacoustic and conductivity measurement forms the basis for our present claim of a new method of ζ-potential characterization, applicable to a wide variety of dispersions and emulsions.